once in a blue moon

Rarely, once in a very long time, as in We only see our daughter once in a blue moon. This term is something of a misnomer, because an actual blue moon-that is, the appearance of a second full moon in the same calendar month-occurs every 32 months or so. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]

once in a blue moon

If something happens once in a blue moon is happens only very rarely. I only get over to Cambridge once in a blue moon and I'm never in London.Only once in a blue moon do properties of this quality become available. Note: In some places, if there are two full moons in a calendar month, the second is called a `blue moon'. This is rare.

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